The operation of such apparatus is based on the Archimedes principle. A measuring element is immersed in the fluid to be measured. When an actuating mechanism is operated, the measuring element is raised to a given height. During its ascension, a vacuum is created in the cavity of a cup in which the lower end of the measuring element is contained. The measuring element is maintained in the raised position during an adequate period of time so as to ensure that the cup cavity is completely filled. When the actuating mechanism is released, the measuring element falls under gravity and, after having travelled a given distance, a detector is actuated to detect the end of the cycle. The time elapsed is proportional to the viscosity of the fluid.
Current systems utilize a piston which has a cylinder at its lower end that is displaced in a cylindrical cup; such systems have two major drawbacks. First, often, during their operation, the measuring units are subjected to heavy mechanical shocks (for example, they may accidently be dropped on a cement floor). If one considers that a distance of at least 0.007 inch usually separates the measuring cylinder from the cylindrical cup wall within which it is housed, any minor misalignment between these two parts will greatly affect measurement readings. This device is therefore extremely vulnerable.
A second problem, more technical, concerns fluid mechanics. If one desires to have a fluid to circulate with a given flow through a gap having a constant diameter and a given length, a certain pressure must be applied to evacuate the fluid. If the length of this gap is increased, while maintaining the same flow, the pressure must be increased. Therefore, by using a piston with a cylindrical head, the length of the gap will increase as the piston descends into the cylinder. Since the pressure is constant (weight of the cylinder and constant gravity pull), the fluid will have increasing difficulty escaping through the gap. This will result in a reduction of speed which is exponential as the piston descends. It has been noted that, at very low speed, this concept becomes inaccurate and unstable, which therefore results in the extremely important need to maintain a constant speed during the entire measuring process.